In the field of AC power conversion a technology for transforming single phase AC voltage to three phase AC voltage has existed in the form of rotary conversion. The rotary converter is known to be a very reliable and durable mechanism for achieving the conversion of single phase voltage into three phase voltage. The benefits of operating equipment on three phase power have long been recognized and appreciated. Particular as it relates the operation of equipment having electric motors. However, the availability of three phase power from a utility can be a problem, particularly in residential and rural areas or locations already provided with single phase power. To overcome this problem various methods are available for converting single phase power into three phase power. These include static converters, electronic converters and rotary converters. In many situations, rotary converters are the device of choice, particularly in view of economics, reliability and flexibility.
The rotary converter has many similarities to a three phase induction motor. The spatial distribution of windings around the periphery of the motor establishes a phase displacement between each of the windings. This phase displacement allows a three phase motor to develop torque when powered by three phase displaced voltages. Similarly, if a three phase motor structure is caused to rotate and is simultaneously energized with a single phase voltage, the additional windings on the motor will generate a voltage with a phase displacement from the energized winding. This action of generating a phase displaced voltage may be used to provide three phase power to an electrical load, such as a motor load, from a single phase source.
The rotary converter has many advantages over other methods of power conversion and a few disadvantages. Rotary converters are very robust and durable. They are tolerant of a wide range of operating conditions including overloads and short circuits, variations in temperatures and fluctuations in output load and input line conditions. Additionally, they do not require moving electrical contacts which are subject to wear, such as brushes or commutators. However, rotary converters do have a number of disadvantages which require consideration when applying these devices. First, the rotary converter does not automatically start itself rotating. It requires electrical or mechanical means to get the rotor of the converter spinning. These techniques include the use of either a single phase starter motor which spins the rotor of the converter before single phase power is applied to the drive winding of the converter or the use of capacitors and switches to impart a phase displaced current in the windings of the rotary converter which cause the rotor to start spinning.
During this starting process it is necessary that the three phase load on the converter be disconnected from the converter and that only the rotary converter is powered. After the rotor is up to speed, the converter is then electrically connected to the desired three phase load or loads. Thus the starting procedure for rotary converters is a multi-step process. After the rotary converter is up and running, it must be left running for the duration in which three phase power is required by the connected load. However, due to the previously mentioned starting sequence, three phase rotary converters are often left running for long periods of time when no electrical load actually requires power. This avoids the need to go through the starting sequence with each usage. However, this has two significant disadvantages. First is an economic issue and the second is an acoustic noise issue.
The rotary converter requires electrical power to run and spin the rotor even when no electrical three phase load is connected. A typical 25 hp rotary converter may consume about 2 kilowatts of power just idling with no connected electrical load. This use of electrical power is a waste of energy and has no value. Additionally, the rotary converter makes acoustic noise similar to that of an electric induction motor. When the three phase load or loads are disconnected from the converter, the background noise of the idling converter can be quite undesirable.